Nocturnal and diurnal foraging behaviour of brown bears (Ursus arctos) on a salmon stream in coastal British Columbia

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Brown bears (Ursus arctos) have been reported to be primarily diurnal throughout their range in North America. Recent studies of black bears during salmon migration indicate high levels of nocturnal foraging with high capture efficiencies during darkness. We investigated the extent of nocturnal foraging by brown bears during a salmon spawning migration at Knight Inlet in coastal British Columbia, using night-vision goggles. Adult brown bears were observed foraging equally during daylight and darkness, while adult females with cubs, as well as subadults, were most prevalent during daylight and twilight but uncommon during darkness. We observed a marginal trend of increased capture efficiency with reduced light levels (day, 20%; night, 36%) that was probably due to the reduced evasive behaviour of the salmon. Capture rates averaged 3.9 fish/h and differed among photic regimes (daylight, 2.1 fish/h; twilight, 4.3 fish/h; darkness, 8.3 fish/h). These results indicate that brown bears are highly successful during nocturnal foraging and exploit this period during spawning migration to maximize their consumption rates of an ephemeral resource.

Les ours bruns (Ursus arctos) sont généralement reconnus comme des animaux à alimentation surtout diurne dans toute leur aire de répartition. Les résultats d'études récentes sur les ours noirs durant la migration des saumons indiquent qu'ils font une quête de nourriture intense pendant la nuit et que l'efficacité de leurs captures est élevée à l'obscurité. Nous avons étudié l'importance de la quête nocturne de nourriture chez des ours bruns au moyen de jumelles de nuit au cours d'une migration de fraye de saumons, à Knight Inlet, sur la côte de la Colombie-Britannique. Les adultes recherchent leur nourriture aussi bien la nuit que le jour, mais les femelles avec un petit et les individus sub-adultes se nourrissent surtout durant le jour et au crépuscule, mais rarement durant la nuit. Nous avons observé une tendance marginale vers une efficacité accrue des captures en fonction inverse de l'intensité lumineuse (jour, 20 %; nuit, 36 %), probablement à cause de la diminution du comportement évasif des saumons. Les taux de capture moyens étaient de 3,9 poissons/h et ils variaient selon le régime photique (jour, 2,1 poissons/h; crépuscule, 4,3 poissons/h; nuit, 8,3 poissons/h). Ces résultats indiquent que les ours bruns font une quête active nocturne très profitable et ils exploitent cette période durant la migration de fraye des saumons de façon à maximiser leur taux de consommation d'une ressource éphémère.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: August 1, 2002

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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